Isoelectric Point Calculator
Use our isoelectric point calculator to calculate pI from pKa and pKb. Includes what isoelectric point is, what the isoelectric point tells you, and the isoelectric point formula used for simple amphoteric molecules.
What Is Isoelectric Point?
The isoelectric point (pI) is the pH at which a molecule has a net charge of zero. At the isoelectric point, positive and negative charges balance out overall.
Isoelectric point is commonly discussed for amino acids, peptides, and proteins. It helps predict behavior in an electric field and solubility changes across pH.
Isoelectric point is not the same as pH. pH describes the acidity of a solution, while pI is a property of the molecule that tells you at what pH it becomes neutral overall.
Isoelectric Point Formula
For a simple amphoteric compound with one acidic group and one basic group, the isoelectric point can be approximated as the average of pKa and pKb-derived acidity constants used for the neutral form. In this calculator, pI is computed as the average of the two input values.
This is a simplified calculation intended for cases where pI is approximated from two relevant dissociation constants.
How to Calculate Isoelectric Point
- 1
Enter the pKa value.
- 2
Enter the pKb value.
- 3
The calculator averages the two values to return the isoelectric point (pI).
Frequently Asked Questions
No. pH describes the solution. pI describes the molecule and tells you the pH where its net charge is zero.
It tells you the pH where a molecule is overall neutral. This can help predict migration in electrophoresis and how solubility changes as pH moves away from pI.
For amino acids, pI is typically the average of the two pKa values that surround the neutral form. Amino acids with ionizable side chains require selecting the correct two pKa values based on charge states.
When a molecule has three ionizable groups (like some amino acids), you identify the charge state transitions and average the two pKa values that bracket the net-zero charge form.
A protein’s pI depends on all ionizable residues and termini. It is often estimated computationally by summing charges across pH and finding the pH where net charge is zero.